Frequency comparison rate servo



April D. L. cuRTxs 2,932,778

FREQUENCY COMPARISON RATE SERVO original Filed Dec. 24, 195e [WUI 2 Sheets-Sheet 1 April l2, 1960 D. l.. CUR-ris FREQUENCY COMPARISON RATE SERVO Original Filed Deo. 24, 1956 United States Patent FREQUENCY COMPARISON RATE SERV() Daniel L. Curtis, Manhattan Beach, Calif., assigner to Litton Industries of California, Beverly Hills, Calif.

13 claims'. (c1. sis-314) The present invention relates to a rate servo for maintaining the speed of a moveable element proportional to the frequency of standard frequency signals and more particularly to a rate servo wherein a moveable element is accelerated or decelerated according to the phase and frequency difference between the standard frequency signals and the variable frequency signals, produced by the moveable element. This application is a continuation of my copending U.S. patent application Serial No. 630,261, filed December 24, 195 6.

It `has been frequently found necessary to maintain the speed of a moveable element proportional to some standard frequency. A few of the more common applications of this form of speed control are in magnetic recording devices, either in the audio or video field, in tracking systems, and especially in controlling a magnetic drum speed in computer work. In computer work, for example, it is necessary to maintain the frequency of revolution of a magnetic drum in synchronism with standard frequency signals such as those obtained from the operation of a tuning fork. l

In the prior art, the magnetic drum is synchronized by one of two methods. According to the first method, standard frequency signals generated from a tuning fork are first amplified by a power amplifier and applied to a synchronous motor to rotate the motor at a frequency equal to the frequency of the applied signal. This system, however, has several severe limitations.

It is a well-known fact that a synchronous motor is less efficient than other types of motors and therefore requires more power to provide a specified drum speed than such other types of motors. This power must be provided by the power amplifier, as it is clear that the tuning fork produces a signal of little or no will of necessity be large, heavy and relatively expensive. In most applications, and particularly in computers designed for airborne use, the advantages of small size and light weight cannot be overemphasized.

In addition, the synchronous motor, being less efficient, produces more heat in operation, thereby heating the surrounding area to a greater degree, than if some other type of motor were used. Therefore, the equipment surrounding the motor may be damaged by the excessive heat, unless means for cooling the area are provided.

As was stated earlier, size and weight are critical with respect to airborne computers. In order to reduce the size and weight of nonairborne computers, as well as airborne computers, the present trend in the art is to miniaturize computers by designing computers using transisters, instead of conventional vacuum tubes. The power amplifier system, hereinbefore described, does not, however, lend itself readily to miniaturization, due to the fact a power amplifier capable of producing the required amounts of power is difiicult to mechanize using transistor techniques known to the art.

The second method in which the prior art synchronized a magnetic drum with respect to a standard frequency is power. A power ampli-V iier capable of producing the required amount ofpower tli) ice

by means of a phase servo system. The phase servo system is responsive to standard frequency signals and variable frequency signals whose frequency is proportional to the speed of the drum, to produce alternate acceleration and deceleration signals, the relative duration of the acceleration or deceleration signals with respect to the other depending upon the phase of the variable frequency signals with respect to the standard frequency signals.

The phase servo system, in its most common embodiment, is mechanized by the use of a flip-flop circuit, which is a well-known circuit in the art. The standard frequency signals are applied to one input of the iiip-op circuit, while the variable frequency signals are applied to the other input. A bilevel voltage signal willbe produced at one output terminal of the Hip-Hop circuit, the duration of one voltage level to the other being dependent upon the phase relationship of the variable frequency signals with respect to the standard frequency signals.

When the frequency of the variable frequency signals the standard frequency signals are equal and out of phase by the duration of one level of the dip-flop output signal will be equal to the duration of the second level. If one level is thought of as an acceleration signal and the other level as a deceleration signal, it can be seen that when the frequency of the Variable frequency signals equals that of the standard frequency signals the phase servo will produce alternate acceleration and deceleration signals of equal duration, therefore, holding the drum, whose speed is proportional to the variable frequency signals, in synchrenism with the standard frequency signals.

if a disturbance should occur, such as would cause the frequency of the variable frequency signals to increase slightly, the phase of the variable frequency signals would advance slightly with respect to the standard frequency signals. The standard frequency signals and variable freqoency signals are applied to the inputs of the tlip-op circuit in such a manner that this phase change will cause tie duration or" the deceleration signal to increase and t of the acceleration signal to decrease, therefore, tending to slow down the drum and, in turn, the frequency of the variable frequency signals.

if, however, a disturbance should occur that would cause a frequency change in the variable frequency signais such that the phase of the variable frequency signals would advance or be retarded more than 180, the phase servo system loses its ability to sense the direction of error. ylierefore, the system under these conditions would apply erroneously directed acceleration or deceleration signal, such that the system may never again be able to synchronize the drum. Even if synchronism is again achieved the drum will have lost several revolutions with respect to the standard frequency signals. The phase servo sys. tem is, therefore, of limited utility when even a moderate degree'of accuracy is required.

in contrast to the prior art, the present invention provides a relatively simple rate servo system that can maintain the speed of a moveable element, such as a motor shaft, in synchronisrn with the frequency of standard frequency signals, no matter how great the difference in frequency between the standard frequency signals and the variable frequency signals. In addition, the system, according to the invention, is not restricted to the use of a synchronous motor but can use any type of motor to drive the moveable element, for example, an induction motor. Further, according to a preferred embodiment of the invention, the synchronizing system can be mechanized by the use of two flip-flop circuits and a small number of gating circuits, rather than a relatively large power amplitier.

According to the basic concept of the present invention,

Vthe rate servo system is responsive to the standard frequency signals and the variable frequency signals to produce acceleration and deceleration signals when the ratio of 'thefrequencies of the variable frequency signals and the standard frequency signals is less than and greater than a predetermined number, respectively. More particularly, according to preferred embodiments of theinvention, the servo system produces acceleration and deceleration vsignals when the frequency of the variable signals is less and greater than that of the standard signals, respectively.

e The preferred method by which the invention accomplishes this result is as follows. The variable frequency'signals and standard frequency signals are made up of sharp electrical impulses so that when the frequency ofthe variable frequencyY signals, for example, exceedsthe frequency ofthe standard frequency signals, a larger number of electrical impulses of the variable Vfrequency signals must occur, per unit time, than of the standard frequency signals. Therefore, there will be intervals when two sharp electrical impulses of the variable frequency signals will occur within the time lapse between the occurrence of sharp electrical impulses of the standard frequency signals. Accordingly, by making the rate servo of the invention responsive to two successive variable frequency signals to produce a decelerating signal and to twosuccessive standard frequency signals to produce an accelerating signal, the moveable element is controlled in suchva manner as to become and remain synchronized with respect to the standard frequency signals.

However, when the frequency and phase of the variable frequency signals and the standard frequency signals are equal, the sharp electrical impulses of the two signals occur simultaneously. This may cause the rate servo of the invention to become ambiguous in its operation and to produce an erroneous acceleration or deceleration signal.`

For extremely accurate synchronization work a modified rate servo system, according to the invention, can be made to operate in such a manner that extremely accurate synchronization can be maintained even when the variable frequency and standard frequency signals are equal or nearly equal in both phase and frequency.

In a modified embodiment of the invention which is described in the present specification, a rate servo system is provided that is relatively simple in structure and which will remove this erroneous acceleration or deceleration signal set forth above. According to this embodiment of the invention, the system responds to the variable frequency signals and the standard frequency signals to provide acceleration and deceleration signals whose durations are related to the phase of the variable frequency signals' with respect to the standard frequency signals when the phase and frequencies of the two signals are equal and thereafter; and to the frequencies of the two signals when the frequencies of the two signals are substantially different and thereafter.

The rate servo system according to the modified embodiment of the invention, therefore, has two modes of operation, a frequency mode and a phase mode. This dual mode, phase-frequency servo is operable in the frequency mode of operation, when two successive variable frequency signals or two successive standard frequency signals occur and in a phase mode of operation, when the standard frequency signals and the variable frequency signals occur simultaneously. The dual mode, phasefrequency servo, further, is operable to switch back and forth from one'mode of operation to the other, when the proper conditions for operation in a particular mode occur. f

It is therefore an object of the present invention to .provide a rate servo for driving a moveable element at a velocity proportional to the frequency of applied stand- Yard frequency signals. V

It is another object of the invention to provide a dual mode, phase-frequency rate servo.

A further object of the invention is to provide a dual mode, rate servo for driving a moveable element at a velocity proportional to the frequency of applied standard frequency signals. e Y

Still another object of the invention is -to provide a dual mode, phase-frequency rate servo for controlling the power applied to a motor to maintain the frequency and phase of the motor shaft constant relative to applied standard frequency signals.

The novel features which are believed to be characteris-tic of the invention, both as -to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a partly block, partly circuit diagram of a preferred embodiment of a rate servo, in accordance with the present invention;

Fig. 2 is a partly block, partly circuit diagram of a modified embodiment of the rate servo, shown in Fig. l;

Figs. 3 and 4 are Acomposite diagrams of waveforms of the signals produced by the embodiment of the rate servo shown in Fig. 2; and

Fig. 5 is a. partly block, partly circuit diagram illustrating a modication of the rate servo'shown in Fig. 2.

Referring now to the drawings, wherein like or corresponding parts are designated by the same reference characters throughout the several views, there is shown in Fig. l a frequency form of rate servo, according to ythe invention, which is operative to drive a moveable element 11 at a speed proportional to the frequency of constant frequency signals S, emitted by a standard frequency signal generator 12. It will be recognized by those skilled in the art that the standard frequency signal generator may embody any of several well-known structures for generating such a signal, as for example, a tuning fork. Y

As shown in Fig. l, the rate servo includes four basic components: a variable frequency generator 16 which is coupled to moveable element 11 for generating variable frequency signals Vf, whose frequency is proportional to the vspeed of the moveable element 11; an actuator 13 which is responsive to the variable frequency signals Vf and the standard frequency signals Sg `for producing rei sultant actuating signals designated A and B, respectively;

vmay be the output shaft of a motor 17. Variable a controller'14'which is responsive to actuating signals AA and Bv for producing a control signal Q2 having either Va high or low level in accordance with the applied actuating signals; and an, accelerator 15 which responds to signal Q2 by accelerating or decelerating moveable element 11 in accordance with the high or low level of control signal Q2.

As specifically shown in Fig. 1, moveable element 11 frequency generator 16 may then include a drum'l rwhich is coupled to the motor shaft `11 in such manner that the drum rotates in synchronism with the motor shaft. Placed on the drum are one or more equally spaced magnetic spots and a magnetic pick-up head 9 which is mounted next to the drum in such a manner that the spot or spots on therdrum pass adjacent the head. When a magnetic spot passes the magnetic pick-up head ay pulse is generated bythe head which is applied to a signal amplifier 8, Vsignal amplifier 8 functioning to amplify and shape Y l VAs shown inFig. l, variable frequency signals Vf and l signal Q1 will be at a high level.

D standard frequency signals S1 are applied to actuator 13 over correspondingly designated conductors V1 and S1, respectively. (For purposes of facilitating and clarifying description, each conductor will be hereinafter similarly designated in terms of the signal applied over the conductor.) Actuator 13 functions to produce actuating signals A as sharp negative pulses on output conductor A when the frequency of signals Vf is greater than the frequency of signals S1, and to produce actuating signals B on output conductor B when the frequency of signals V1 is less than the frequency of signals S1. More particularly actuator 13 functions to produce a signal A whenever two successive V1 impulses are received and a signal B whenever two successive S1 signals are received. Controller 14 is responsive to each actuating signal A for producing signal Q2 at its high voltage level and is responsive to each actuating signal B for producing signal Q2 at its low level. the high or low level of signal Q2 by accelerating motor shaft 11 with a high or low magnitude acceleration, respectively.

Referring now with particularity to the structure and operation of actuator 13, as shown in Fig. 1 actuator 13 is seen to comprise a fiip-op Q1 and a pair of gates G3 and G1. Flip-hop Q1 has a pair of inputs designated as the S input and Z input respectively, each input being further designated by the numeric subscript 1 corresponding to the numerical designation' of the hip-flop. Flip-flop Q1 produces an output signal Q1 having high or low voltage levels in accordance with the state of the Hip-flop and also produces an output signal Q1 having opposite or complementary voltage levels.

in operation flipflop Q1 is responsive to the application of an input signal to its S input conductor for being set to its set state and to the application of an input signal to its Z input conductor for being set to its zero state. When ilip-ilop Q1 is in its set state, signal Q1 will be at its high level while complementary signal Q1 will be at its low level. Conversely when iiipdiop Q1 is in its ,zero state, signal Q1 will be at a low level while complementary The detailed structure for one suitable form of tlipdlop can be found in U.S. Patent No. 2,733,430 issued January 3l, 1956, for Angular Quantizer, by Floyd G. Steele.

Gates G3 and G4 may be identical in stmcture and function. Gates G3 and G4 can be any one of the wellknown types of pulse operated and gates for passing an impulse under the control of the high or low voltage levels of an applied signal. `As shown in Fig. 1, each and gate includes two inputs and a single output and is responsive to a high level voltage signal applied to one input and'to a sharp negative pulse applied to the other input conductor, to produce a sharp negative pulse at the output of the gate.

As shown in Fig. 1, gate G4 includes a diode 7l), whose cathode is connected tothe input conductor Q1v and whose anode is connected to a terminal '71 and is also coupled through a pull-up resistor 76 to a source of relatively high voltage B+. The anode of a diode 72 is connected to common terminal 71 and the cathode of diode 72 is connected Vto terminal "i3 and is also coupled through a pulldown resistor 7S to a source of relatively low voltage B. The cathode of a diode 74 is connected to input conductor S1, while the anode is connected to terminal 73 and one terminal of capacitor 80, the other terminal of capacitor 80 being connected to output conductor B.

As shown in Fig. l, terminal 73 will be at a high or low voltage level when signal Q1 is at a high or low voltage level, respectively. The rate at which the voltage at terminal 73 changes when signal Q1 changes its voltage level is limited by the size of pull-up resistor 76 and pull-down resistor 78 in such a manner that the resultant diterentiated pulse formed by capacitor S in response to these voltage changes will he small enough so that flip-flop Q1 will not be triggered thereby. When an S1 signal is ap- In turn, accelerator 15 responds to plied to the cathodeof diode 74, the S1 signal will rabruptly pull-down the voltage at terminal 73 if that voltage is at a high level and will leave it unchanged if the voltage is at a low level, as it cannot be made to go any lower. Thus when the voltage at terminal 73 is at a high level and is abruptly pulled down by the S1 pulse, capacitor 80 will form a sharp negative pulse which will trigger lli ilop Q2, while if the voltage at terminal 73 is low at the time S1 is applied, capacitor S0 will produce no output pulse and flip-hop Q1 will not be triggered. Gate G3 operates in the same fashion to produce a negative output pulse when a high level signal is present at one input simultaneously with the application of a V1 signal at the other input.

As shown in Fig. 1 pulse signals Vf are applied both to the S input of iiip-op Q1 and to one input of gate G3 `f/hile pulse signals S1 are applied to the Z input of ilipilop Q1 and also to one input of gate G4. The bilevel dip-flop output signals Q1 and Q1 are applied to the other inputs of gates G3 and G4, respectively. Gates G2 and G4 are responsive to these applied input signals for respectively producing the actuating signals A and B, which as hereinbefore described are applied to controller 14.

As shown in Fig. l, controller 14 comprises one major component, a ilipdop Q2 which is operable in the same manner as iiip-flop Q1. Flip-flop Q2 has a pair of inputs designed as S2 and Z2 and is responsive to signals applied to these inputs for producing as its output the control signal Q2 and also a complementary output signal Q2. it is seen, referring to Fig. l, that actuating signals A are applied to the Z2 input of flip-flop Q2 while actuating signals B are applied to the S input of Hip-flop Q2. In operation, it is clear that controller 14, responds to each application of an actuating signal A or an actuating signal B for producing control signal Q2 at its low and high voltage levels respectively, control signal Q2 as hereinbefore described being applied to accelerator 15.

Accelerator 15 as illustrated in Fig. l, comprises a motor 17, and a power regulating circuit 6 for regulating the amount of power delivered to the motor from` an external current source not shown in accordance with the high or low voltage levels of control signal Q2.

Motor 17 includes a motor coil 44 and a motor coil 2d having a common input terminal 51 and output terminals designated 52 and 53,V respectively. Power regulating circuit 6 comprised a diode 42, a capacitor 19 and a transistor having a base electrode 46, a collector electrede 48, and an emitter electrode 50. As shown in Fig. l conductor Q2 is connected to base electrode 46 and emitter is connected to a source of ground potential while collector 48 is connected to one side of capacitor i9, the other side of capacitor 19 being connected to terminal 52 of motor winding 20. Diode 42 is connected between collector 4S and emitter 50 (and thus to the source of ground potential). Terminal 53 of winding 44 is also connected to the source of ground potential.

In operation, assuming that alternating current is Supplied to the current source, it is clear that current will continuously tlow through motor coil 44 at a constant rate to the source of ground potential, thereby producing a constant low level accelerating force on motor shaft 11. Additional accelerating force may be provided by current ilowing through motor coil 2u and then through power regulating circuit 6 to the source oi' ground potential. It is clear that if transistor 40, within power regulating circuit 6, is biased oi (non-conductive) by signal Q2 having a low level (assuming transistor' 40 is an NPN transistor) current can pass only through diode 42, during `the positive half cycle of the current, and then only until capacitor 19 becomes fully charged, at which time all current ow through motor coil 2i) will cease. However, if transistor 49 is biased conductive by signal Q2 at its high level, transistor 40 will be capable of passing the negative half cycle of the A.C. signal, the positive half cycle passing as before through diode 42, so that full current flows through coil 20. Therefore, when transistor 40 is biased by signal Q2 at a high level, current will flow through both motor coils 2b and 44 thus producing a high level accelerating force upon motor shaft 11, while when transistor `46* is biased off by signal Q2 at its low level, current will iiow only through motor coil 44 and a resultant low level accelerating force will be applied to motor shaft 11.

Itis therefore clear that in overall operation accelerator responds to the high `or low voltage levels of signal A for applying a corresponding high or low level accelerating force to motor shaft 11 to thereby accelerate shaft 11 to a relatively high velocity or to allow motor shaft 11 to be decelerated (by frictional forces and other effects) to a relatively low Velocity.

It will be recognized by those skilled in the art, that the particular embodiment of power regulating circuit 6, shown in Fig. 1 is only one of many structures whereby the flow of power to motor 17 may be regulated by control signal Q2 and it is not intended that the invention should be limited to the particular power regulating circuit shown. For example, a coventional relay circuit may conveniently be utilized to control the power applied by motor 17 or in other embodiments an eddy current brake or negative clutch may be used to control the power delivered by motor 7 to shaft 11.

Referring now with particularity to actuator 13, as hereinbefore mentioned, dip-riep Q1 receives standard frequency signals S1 at its Z'input and variable frequency signals V1 at its S input. It will be remembered that when iiip-flop Q1 is in its zero state, signal Q1 will be at a low voltage level thus blocking the passage of any V1 pulse signals through gate VG3 since as hereinbefore described gate G3 will pass signals V1 to form resultant actuating signals A only when a high level voltage signal is applied to gate G3 over conductor Q1.

It is well-known that in the operation of a flip-hop, a certain delay time is introduced between the time a set or zero signal is received and the actual change in state of the flip-liep. It is therefore clear that if flip-flop Q1 is initially in its zero state at the time a V1 signal is applied, that V1 signal will not be passed through gate G3 since at the time the V1 signal arrives at gate G3, flipflop Q2 will not yet'have changed from its set to its Y zero state so that signal Q1 will be at alow level blocking the passage of the V1 signal. Accordingly, it is clear `that a V1 pulse will be passed through gate G3 to form an actuating signal A only when flip-flop Q1 is initially in a set state preceding the application of the V1 signal to the actuator. Since as hereinbefore mentioned, a V1 pulse must be applied to actuator 13 in order to set liip-op Q1, i-t is therefore clear that two successive V1 signals must be received by actuator 13 in order to produce an actuating signal A from gate G3. By the same reasoning it can be shown that two successive S1 signals must be applied to actuator 13 to produce an actuating signal B from gate G4.

Continuing further with the description of operation, as hereinbefore described, each actuating signal B will set nip-flop Q2 to its set state, thereby raising signal Q2 to its high level at base electrode 4.6 of transistor 40. On the other hand, each actuating pulse A will cause flip-flop Q2 to be zeroed, therefore causing signal Q2 to be at its low level at base electrode 45 within accelerator 15. In response thereto, accelerator 15 applied high accelerating forces to motor shaft 11 when signal Q2 is at a high voltage level, and applies relatively low accelerating forces to motor shaft l1 when signal Q2 is at a low level.

It can be seen, then, that two successive standard frequency pulses S1 will cause more power to flow to the motor and thus accelerate moveable element 11. the other hand two successive variable frequency signals V1 will cause less power to flow to the motor and a deceleration of moveable element i1 will occur. Actuator will receive two successive standard frequency signals only when the frequency of variable frequency signal is less than that of the standard frequency signal,

indicating that the velocity of moveable element 11 is too low. Thus it is clear that the resultant high accelera- `tion applied has the correct effect of speeding up moverate servos. However, it should be understood that when the phase and frequency of the standard frequency signal and the variable frequency signal become substantially equal because of inability of flip-flop Q1 to resolve the nearly coincident pulses, there is a possibility that an incorrect A or B signal may be occasionally emitted causing a positive feedback signal -to the motor and a resultant momentary loss of exact synchronism. rl`herefore, for applications of rate servo requiring an extremely high degree of accuracy, a modified form of the rate servo of the present invention, a dual mode, phasefrequency form of rate servo described hereinbelow isk preferably utilized.

Referring to Fig. 2, there is shown a dual mode phasefrequency form of rate servo, according to the invention, wherein the frequency form of rate servo described hereinabove is modified to a dual mode phase-frequency form of rate servo which basically operates as a frequency Yform rate servo until the frequency and phase of the variable frequency signals and the standard frequency signals near the point at which they arc substantially equal, at which time the dual mode phase-frequency form of rate servo will change its form of operation so that i-t thereafter operates as a phase servo. if later the .frequency of the variable frequency signal and the standard frequency signal again become substantially unequal for some reason, as for example in response to a disturbance in the current source the phase-frequency form of rate servo will at that time again change back to a frequency form of operation. As a result of this dual mode operation of the servo, the possible occasional ambiguities associated with the pure frequency form of rate servo (at substantial coincidence of S1 and V1 signals) are entirely eliminated, while all of the desirable features of the described frequency forrn of rate servo (for substantial deviations between S1 and V1 signals) are retained.

The dual mode phase-frequency servo shown in Fig. 2 is modified with respect to the frequency form of rate servo of Fig. l by the addition of apparatus for feeding back the output signals produced by controller 14 as additionalinputs to actuator 13. From the additional information provided by these feedback signals, actuator 13 is enabled to change its mode of operation from frequency comparison to phase comparison and vice versa at the proper times.

As illustrated in Fig. 2, the output signals Q2 and also a complementary output signal Q2 of controller 14 are fed back to actuator 13 over corresponding conductors 21 and 22, signals Q2 and Q2 being applied within actuator 13 to the inputs of a pair of differentiating circuits 26 and 25 respectively to form corresponding differentiated output signals Q2 and Q'2, respectively. As illustrated in Fig. 2, the differentiated signal @'2 are mixed in a conventional mixing circuit 23 with signals V1 for application to the S input of flip-flop Q2 while the differentiated signals Q2 are mixed in another mixing circuit 24 for application to the Z input of iiip-flop Q1.

. Dilferentiating circuits 25 and 26 function to produce sharp negative pulses Q'2 and Q2, respectively whenever the corresponding input signal Q2 or Q2 change from a high to low voltage level. The operation and structure of such differentiating circuits well-known to one skilled in the art.

It is clear in view of the foregoing that signal Q2 will be applied to the S input of flip-flop Q1 to set flip-dop Q1 whenever Hip-liop Q2 changes from its zero state to its set sta-te while signal Q'2 will be applied to the Z input of dip-liep Q1 to Zero ilip-flop Q1 whenever ilipop Q2 changes from its set state to its zero state.

Referring now with particularity to the operation of the dual mode phase-frequency form of rate servo shown in Fig. 2, it will be understood that until the phase and frequency of the variable frequency signals and the standard frequency signals are substantially equal, the dual mode phase-frequency form of rate servo system will function in yits frequency mode of operation. An illu..- tration of the operation of the phase-frequency form of the rate servo near and at the point where the phase and frequency of the variable frequency signals and the standard frequency signals are equal, refer to Fig. 3, wherein, is shown a series of standard frequency signals S1 and variable frequency signals V1 plotted on a common time scale. There is also illustrated in Fig. 3 on the same time scale the corresponding Wave-forms of output signals Q1 and Q2 of flip-flops Q1 and Q2, respectively. Assuming as shown in Fig. 3 that the frequency of the V1 signals is initially much less than the frequency of the S1 signals, then as hereinbefore mentioned the dual mode phase-frequency form of rate servo will operate in its frequency mode. As shown at time T11 of Fig. 3, hip-flop Q2 is initially assumed to be in its set state so that signal Q2 is at a high level causing motor 17 to accelerate moveable element 1l. 1

As described in connection with Fig. l, as long as two successive S1 signals occur between each pair of signals V1, tlipdlop Q2 will remain se, thereby maintaining signal Q2 at a high level, while flip-flop Q1 will alternate from one of its states to the other. As hereinbefore mentioned, signal Q2 at its high level will cause moveable element 11 to be accelerated, thereby causing the frequency of the V1 signals to increase, as is shown in Fig. 3. As the frequency of the V1 signals increases, a point will be reached where the frequency and phase of the two signals are near equal, as shown at point T of Fig. 3. At this point the dual mode phase servo commences to change over to its phase form of operation.

Directing attention to Fig. 3, there is shown a V1 signal occurring at time T and an S1 signal occurring in near coincidence with the time TV1 signal. Assume that fliptlops Q1 and Q2 and in their zero and set states, respectively, before the occurrence of the time TV1 signal, as shown in Fig. 3. It will be understood that whenever an S1 and V1 signal are applied to flip-liep Q1 in near coincidence, the dip-flop will change its state Without regard to its prior state or to whether the S1 or V1 signal occurs rst. Therefore, when the S1 signal occurring at time T and the nearly coincident V1 signal is applied to flip-Hop Q1 the flip-flop perforce changes from its zero state to its set state.

Thereafter at time T1, very nearly coincidental V1 and S1 signals are again applied to flip-hop Q1, and flip-dop Q1 again changes its state this time from its set state to its zero state. When nip-flop Q1 is zeroed signal Q1 will go from a high level to a low level but since it will take a finite period of time for this change to take place, signal Q1 will still be at a high level when the following V1 signal is applied to gate G3. As hereinbefore mentioned gate G3 will in response thereto emit a negative activating signal A, which is applied to the Z input of Hip-flop Q2 to cause ilip-op Q2 to be zeroed thererby causing a decelerating force to be applied to moveable element 11.

Since vflip-flop Q2 is changed from a set state to a ,zero state, signal Q2 falls from a high level to a low level and as thereinbefore mentioned, differentiator 26 will in response thereto emita negative pulse feedback signal Q2 which is applied to the Z input of hip-flop Q1. However, since flip-dop Q1 is already at its zero state when this feedback signal is applied, the feedback signal has no effect on flip-flop Q1. At this time the dual mode phase frequency rate servo has completely changed its mode of operation from a frequency mode to a phase mode of operation.

As shown in Fig. 3, at any time after time T1, the dual mode rate servo produces signal Q2 at alternate high and low levels, the relative duration of the high and low levels of signal Q2 depending upon the phase of the V1 signals with respect to the S1 signals. For example, refer1 ring now to the S1 signal that occurs at time T2 in Fig. 3, because flip-flop Q1 is at its Zero state signal Q2 is a high level signal. When at time T2 the S1 signal is applied to gate G1, as hereinbefore mentioned, gate G4 will pass it as an actuating pulse B to the S input of flip-flop Q2 to cause flip-flop Q2 to be set, thereby causing differentiator 25 invturn to emit a negative feedback pulse Q32 which is applied to the S input of flip-flop Q1. Flip-flop Q1 is thereby set by the feedback pulse to the same state (the set state) that flip-flop Q2 has just been placed in. in the same manner, when at time T3, the following V1 signal zeroes flip-flop Q2, the resultant feedback pulse Q2 will Zero flip-flop Q1 correspondingly. it is thus seen that in the phase mode of operation, as shown in Fig. 3, liip-op Q1 is slaved to flip-flop Q2 in that it will always change state to agree with new states of flip-flop Q2, due to the feedback signals hereinbefore mentioned. In this condition, the output signals Q1 and Q1 always permit free passage of signals V1 and S1 through gates G3 and G4 to respectively Zero and set flip-hop Q2. Thus during the phase mode of operation flip-flop Q2 will be zeroed by each V1 signal and set by each S1 signal thus casing signal Q2 to have alternate low and high levels in accordance with the occurrence of V1 and S1 signals.

The dual mode system in a phase mode of operation will reach a stable status as shown in Fig. 3, at time T4, holding the frequency of the S1 and V1 signals thereafter equal and maintaining the two signals out of phase by substantially The dual mode phase-frequency form of rate servo will continue in its phase mode of operation until the stable status, hereinbefore mentioned, is disturbed in some manner (as for example, by a current disturbance) to such a degree that either two successive V1 or S1 signals will occur, at which time the dual mode rate servo will change back to a frequency form of operation, as illustrated in Fig. 4.

Referring now to Fig. 4, wherein the waveforms shown, can for purposes of example, be considered an extension of the waveforms shown in Fig. 3, it can be seen that at imes earlier than time T5 the dual mode rate servo continues to operate in its phase mode of operation. How

ever, it is assumed that shortly after time T1, some disturbance took place, as for example, a change in the voltage at which current is supplied, that forced the speed of movable element 11 to increase, thereby increasing the frequency of the V1 signals. Due to the increased frequency of the V1 signals, the period between the V1 sign nals occurring at times T5 and T5 is small enough so that two successive V1 signals will occur and therefore, the dual mode rate servo will change to a frequency mode of operation.

Attention is directed to the time preceding time T5 which it can be seen from Fig. 4, that flip-flops Q1 and Q2 are at their zero states. When the V1 signal occur ring at time T6 is applied to gate G3 signal Q1 is at its low level so that, as hereinbefore described, gate G3 will not emit an actuating pulse and hip-Hop Q2 will remain zeroed. However, since the V1 signal is simultaneously applied to the S input of ilip-op Q1 it is effective for setdessins 11 ting flip-flop Q1. Because flip-flop Q2 has not changed its state no feedback signal can be formed, as hereinbefore mentioned. Thus immediately after time T6 ip-flop Q2 is at its zero state and flip-flop Q2 is at its set state, this having the effect of ensuring, as described hereinbelow that flip-flop Q1 is no longer slaved to flip-flop Q2.

For example, when at time T1 and S1 signal occurs, it will be applied to gate G4, but since signal Q1 is at its low level, gate G4 will not emit an actuating pulse and therefore Hip-flop Q2 will remain in a Zero stateV (indicating in accordance with the frequency mode of operation that the frequency of signals YV1 is greater than the frequency of signals S1).

It can be seen that the dual mode rate servo will thereafter continue to operate in this frequency mode of operation until the phase and frequency of the S1 and V1 signals are near equal.V As shown in Fig. 4, after time T5 iiip-fiop Q2 remains at its zero state, it is thereby acting to decelerate movable element l1 to a relatively low velocity. Therefore the frequency of the'Vf signals will decrease as the speed of movable element il decreases and it will be understood that in continuing operation a point will soon be reached at which the phase and the frequency of the two signals are again nearly equal. At that time the dual mode rate servo system will change again from a frequency mode of operation to a phase mode of operation and maintain the frequency of the V1 signal equal to that of the S1 signal as hereinbefore described in connection with Fig. 3. Thus it is clear that in overall operation, the dual mode phase-frequency form of rate servo functions to increase or decrease the velocity of movable element il at a velocity proportional to the frequency of applied S1 signals with the highest degree of accuracy.

The embodiment of the dual mode phase-frequency form of rate servo hereinbefore described is operable with pulse type S1 and V1 signals. In a few special applications it is desirable to operate with either one or both of the S1 and V1 signals being a signal of long duration. The dual mode phase-frequency form of rate servo hereinbefore described, may be readily modified so that it is operable with signals of relatively long duration, one such modified embodiment being shown in Fig. A5.

Referring now to Fig. 5, there is shown a modified embodiment of the dual mode phase-frequency form of rate servo which is operable with S1 and V1 signals of relatively long duration. As illustrated in Fig. 5, the dual mode phase-frequency form of rate servo shown in Fig. 2 is modified by applying signals Q1 and Q1 to a gate 27 and a gate 28, respectively over a pair of input conductors 3d and 29, respectively. Further, signals S1 and V1 are applied to gates 27 and 2S, respectively, and the output of gates 27 and 2S are applied to its input terminals of mixing circuits 24 and 23, respectively, where they are Vmixed with signals Q2 and Q2, respectively, and applied to the Z and S inputs, respectively of flip-flop Q1. Gates 27 and 2% are high pass pulse gates similar in structure and Afunction to gates G3 and G4, hereinbefore mentioned.

Theunmodiiied embodiment of the dual mode rate servo as shown in Fig. 2, could not operate with S1 and V1 signals of long duration for the reason thatsuch signals applied to the S or Z inputs of ip-op Q1, would interfere with the applicationof the feedback signals Q2 or Q2 to flip-dop Q1. What would occur is that a long duration S1 or V1 signal would still beV present at an input of flip-flop Q1 at the time that an immediately succeeding Q'2 or @'2 signal was fed back, thereby preventing flip-flop Q1 from responding to the fed back signals. This difliculty caused by long duration S1 and V1' signals is eliminated in the embodiment of Fig. 5, by shortening the received V1 and S1 signals by passing them through gates 28 and 29 respectively, these gates functioning as described hereinbelow, to clip off the tail ends of the applied V1 and S1 signals.

Referring now with particularly to Fig. 5, assume that flip-flop Q1 is initially in its Zero state so that signal Q1 is atv its high level, and assume also that the S1 and V1 signals are of a long duration. A V1 signal applied to` gate V2.3 atthis time could pass to the output of gate 28, since signal Q1 at its high level will be present at the other input of gate 28. The passed V1 signalwill then be applied to the S input of flip-dop Q1 and thereby cause the flip-flop to be set However, when flip-hop Q1 is set, signal Q1 changes to its low level state and therefore, signal Q1 is applied to the input of gate 28 at its low level. Now if the V1 signal is of such a long duration that the tail of the signal is still passing through gate 28, the tail will be-clipped-out or cut-off since gate 2S, as-hereinbefore mentioned, can no longer pass a signal. Thus the tail of the V1 signal will not be applied to the input of flip-flop Q1 and therefore cannot mask out a possible feedback signal from flip-Hop Q2.

S1 signals are similarly shortened in duration by passage through gate 27. Each passed S1 signal will be applied to the Z input of flip-flop Q1 and thereby cause the flip-flop to be zeroed However, when flip-flop Q1 is zeroed, signal Q1 falls to its low level, thus cutting off the tail of the S1 signal still passing through gate 27. Thus the tail of the S1 signal will not be applied to the input of flip-flop Q1 and therefore will not mask out a possible feedback signal from flip-flop Q2.

It will be understood, of course, that the rate servo of the invention may be modified or altered in many particulars without'departing from the invention. `For example, although, it was stated hereinbefore that gates G3 and G4 were high pass gates, it is clear that the rate servo of the invention may be modified to operate with low pass gates by merely interchanging conductors Q1 yand Q1. Accordingly, it is to be expressly understood that the invention is to be limited only by the spirit and scope of the appended claims.

What is claimed as new is:

l. in a servo for driving a movable element at a velocity proportional'to the frequency of applied standard frequency signals, the combination comprising: signal generating means for generating variable frequency signals whose frequency is proportional to the velocity of the movable element; first means responsive to said variable frequency signals and the standard signals, said first means being normally operable in a rst mode of operation for producing a iirst signal when the frequency of said variable frequency signals exceeds the frequency of the standard frequency signals and a second signal when the frequency of said variable frequency signals is less than the frequency of the standard frequency signals; second means responsive to said first and second signals for producing a control signal having iirstand second levels, respectively; and accelerating means, responsive to said Icontrol signal at its first and second levels, for accelerating said movable element at predetermined first and second rates, respectively. l

2. The combination defined in claim l which further includes feedback means, coupled between said first and second means, for transmitting a feedback signal to said first means whenever said control signal changes its level yto render said first means inoperable in its first mode of voperation and operable in a second mode of operation, said first means being responsive in its second mode of operation to each standard frequency signal and each variable frequency signal for producing said iirst and second signals, respectively.

3. rThe combination defined in claim 1 which further includes feedback means, coupled between said iirst and second means, for applying said control signal as a feedback signal to said rst means, said first means including apparatus responsive to said feedback control signal for rendering said first means inoperable in its first mode of operation and operable in a second mode of operation.

' first means further includes means operable for producing said first signal when two successive standard frequency signals are received and said second signal when two successive variable frequency signals are received.

8. A dual mode, phase-frequency servo for controlling the power applied to a motor to maintain the frequency and phase of the motor shaft constant relative to applied standard frequency signals, said servo comprising: first means for producing motor shaft signals whose repetition frequency is representative of the speed of the motor shaft and whose phase relative to the standard frequency signals is representative of the phase of the motor shaft;

second means selectively actuable for producing a first or a second output signal; power applying means coupled to the motor and responsive to said first and second output signals for applying power to said motor at rst and second predetermined rates respectively; third means, receiving the standard frequency signals and said motor shaft signals and responsive to two successive standard frequency signals or two successive motor shaft signals, for actuating said second means to produce said first or second output signals respectively, said third means also being responsive to substantially coincidental standard frequency and motor shaft signals for actuating said second means during alternate time intervals to produce first and second output signals respectively, the relative durations of the time intervals being proportional to the phase of the motor shaft signals to the standard frequency signals.

9. The combination defined in claim 8 wheretin said third means includes two and gates and two bistable elements.

10. The combination defined in claim 9 wherein said second means includes one bistable element.

11. The combination defined in claim 8 wherein said motor is an induction motor.

12. A dual mode, phase-frequency servo for controlling power applied Ito a motor to maintain the frequency and phase of the motor shaft constant relative to applied standard frequency signals, said servo comprising: first means, coupled to the motor shaft, for producing motor shaft signals whose repetition frequency is representative of the speed of the motor shaft and whose phase relative to the standard frequency signal is representative of the phase of the motor shaft; second means, receiving the standard frequency signals and the motor shaft signals, said second means being actuable for producing a first output signal in response to two successive standard frequency signals and a second output signal in response to two successive motor shaft signals; third means, receiving the standard frequency signals and said motor shaft signals, said third means being actuable for producing said first and second output signals in alternate time periods, the relative duration of the time periods being proportional to the phase of the motor shaft signals relative to the standard frequency signals; fourth means coupled to the motor and responsive to said first and second output signals for controlling the power delivered to the motor; and fifth means for selectively actuating either said second means .or said third means, said fifth means being responsive to two successive standard frequency signals or two successive motor shaft signals for actuating said second means and responsive to substantially coincidental standard frequency and motor shaft signals for actuating said third means.

13. In a dual mode servo for driving a. movable element at a velocity proportional to the frequency of an applied standard frequency signal, the combination comprising: accelerating means actuable for accelerating the movable element at a rate less than or equal to a maximum rate and greater than or equal to a minimum rate; signal generating means coupled to the movable element for generating a variable frequency output signal whose frequency is proportional to the velocity of the movable element; first means responsive to said variable frequency signal and the standard signal when their frequency difference is less than a predetermined value for actuating said accelerating means to accelerate said movable element at a rate proportional to the phase of the variable frequency signal relative to the standard frequency signal; second means responsive to said variable frequency signal and the standard frequency signal when their frequency difference exceeds the predetermined value for actuating said accelerating means to accelerate said movable element at said maximum or minimum rate.

No references cited. 

